If you know a tiny bit of GDL you could script your own railing post.
You'll need to derive the slanting angle however by yourself. It is given implicitly through a vector stored in the global "RAILINGPOST_TOP_COORD".
With a tiny bit of trigonometry the angle is easy to get and can be forwarded to a "ROTY" command:
tri_a = RAILINGPOST_TOP_COORD
tri_b = RAILINGPOST_TOP_COORD
tri_c = sqr(tri_a^2 + tri_b^2)
alpha = acs((tri_b^2 + tri_c^2 - tri_a^2)/(2*tri_b*tri_c))
!/*.... your geometry comes here */!
Mostly AC 26 on Mac | Author of SelfGDL | Developer of the GDL plugin for Sublime Text
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